1. Field of the Invention
The present invention relates to a method for manufacturing a superconducting device and a superconducting device manufactured thereby. More specifically, the present invention relates to a method for manufacturing a superconducting device by depositing a thin film such as non-superconductor material thin film or a semiconductor material thin film on an oxide superconductor material thin film formed on a substrate, and a novel superconducting device manufactured thereby.
2. Description of Related Art
In the case of using an oxide superconductor material in a superconducting device, it it necessary to form and stack a thin film of oxide superconductor material. For example, when a superconductor/non-superconductor/superconductor junction called a "tunnel type" Josephson junction is formed by using an oxide superconductor material, it is necessary to sequentially stack a first thin film of oxide superconductor material, a thin film of non-superconductor material, and a second thin film of oxide superconductor material in the named order.
In this tunnel type Josephson junction, a thickness of the non-superconductor material thin film is generally determined by a coherence length of the superconductor material. Since the oxide superconductor materials have a very short coherence length, it is necessary to cause the non-superconductor material thin film to have the thickness on the order of a few nanometers.
On the other hand, considering the characteristics of the device, each of the above mentioned three thin films has to have good crystallinity. Namely, all of the three thin films are preferred to be a single crystal, and if any one of the three thin films is polycrystalline or amorphous, a Josephson device cannot have a stable performance.
The good crystalline thin film is required not only in the above mentioned tunnel type Josephson junction device, but also in other devices including a superconducting transistor composed of an oxide superconductor material and a semiconductor material in combination.
However, if the oxide superconductor material thin film is exposed to air, it will lose both superconductivity and crystallinity to the extent of about 1 nm depth from its surface. In ordinary cases, in order to deposit a second thin film on the oxide superconductor material thin film, a deposition apparatus different from that used for formation of the oxide superconductor material thin film is used, and therefore, the oxide superconductor material thin film is inevitably exposed to air in the course of feeding from a deposition apparatus to another deposition apparatus. Because of this, it has been a conventional practice to heat the oxide superconductor material thin film at about 700.degree. C. under an ultra high vacuum on the order of 1.times.10.sup.-9 Torr, before the second thin film is deposited on the oxide superconductor material thin film.
The oxide superconductor material thin film subjected to the above mentioned heat-treatment can have a surface of an improved crystallinity, and in addition, it becomes possible to epitaxially grow the second thin film on the oxide superconductor material thin film. However, the heating of the oxide superconductor material thin film under the above mentioned ultra high vacuum will result in loss of oxygen from the oxide superconductor material crystal thin film, which will deteriorate or lose the superconduction characteristics.
On the other hand, if the heat treatment is performed in oxygen atmosphere, no deterioration will occur in superconduction characteristics of the oxide superconductor material thin film, but the crystallinity of the thin film surface is not improved. Therefore, it may be considered to perform the heat treatment in oxygen or ozone atmosphere after the second film is deposited on the oxide superconductor material thin film. However, even if the oxide superconductor material thin film is treated with this method, after the oxide superconductor material thin film is heated in a later step, oxygen will in some cases be lost again from the crystal of the oxide superconductor material thin film.